1. Field of the Invention
The present invention relates to a secondary battery which can repeatedly be used, and more particularly to a reliable secondary battery capable of preventing short circuits occurring due to dendrites even if the battery is repeatedly charged and discharged.
2. Related Background Art
Since global warming is expected to take place due to the greenhouse effect caused from an increase in CO2 and so forth, construction of thermal power plants has encountered problems. Accordingly, it has been considered feasible to perform so-called load levelling for the pupose of effectively using generators by accumulating electric power at night in secondary batteries at homes to level the load.
Another desire has arisen to development of secondary battery which exhibits a high energy density for use in an electric car that does not exhaust air contamination substances. Further, development of a high performance secondary battery has been needed for use as a power source for portable equipment, such as a book-type personal computer, a word processor, a video camera and a portable telephone.
A locking chair type lithium ion battery capable of serving as the foregoing high performance secondary battery and comprising a positive pole activating material comprising lithium ions introduced into an interlayer compound thereof and a negative pole activating material comprising carbon has been developed and partially put into practical use.
However the lithium ion battery has not achieved the high energy density that is the original characteristic of the lithium battery which uses the metal lithium as the negative pole activating material. The reason why a large capacity lithium accumulator of the type that uses the lithium metal as the negative pole has not been put into practical use is that generation of dendrite of the lithium (tree branch like crystal) which is the main cause of a short circuit cannot be prevented yet.
The lithium battery, nickel-zinc battery and the air-zinc battery encounters the problem that lithium or zinc is, as described above, deposited on the surface of the negative pole at the time of charge. At this time, the current density is locally raised on the negative pole surface depending upon the surface condition, causing lithium or zinc to be selectively deposited in the foregoing place. The deposited metal grows (dendrite) in the form of tree branch with the progress of the charge and discharge cycles while penetrating a separator until it reaches the positive pole, causing a short circuit to occur.
The dendrite reaction mechanism is considered as follows. Since lithium or zinc that deposits at the time of charge has a considerable reactivity, it reacts with electrolytic solution or water or the like in the electrolytic solution, causing an insulating film to be formed which has a large resistance. Therefore, the current density in the foregoing portion is raised at the time of the next charge, resulting in that the dendrite can further easily grow. It leads a short circuit taking place between the negative pole and the positive pole, resulting in that charging cannot be performed.
If the short circuit is extensive the energy of the battery will be consumed in a very short time, causing heat to be generated. As a result, the solvent of the electrolytic solution can be decomposed, resulting in generation of gas. What gas is generated, the internal pressure is raised. In the worse case, an accidental burst or fire can be generated. Therefore, there has been a desire for a long life lithium accumulator that does not easily cause the internal short circuit even if the charge and the discharge cycles are repeated.
Also nickel-zinc batteries and air-zinc batteries generate dendrites of zinc due to repetition of charging and discharging, the dendrites penetrating the separator. As a result the zinc negative pole and the positive pole encounter a short circuit. Therefore, the foregoing conventional technology suffers from an excessively short cycle life.
An object of the present invention is to provide a lithium, lithium alloy, zinc or zinc alloy secondary battery capable of overcoming the foregoing problems experienced with the conventional secondary batteries and exhibiting a long cycle life.
In order to overcome the foregoing problems experienced with the conventional technology, the inventors of the present invention have made energetic studies. As a result, a fact was found that generation of dendrite of lithium or zinc can be prevented by forming a film permitting ions relating to battery reactions to pass through on the surface of the negative pole.
The present invention is characterized in that a secondary battery comprises a negative pole made of a negative pole activating material, a separator, a positive pole made of a positive pole activating material, an electrolyte (electrolytic solution), a collecting electrode and a battery case, wherein the surface of the negative pole is covered with a film permitting ions relating to battery reactions to pass through.
The material of the film has a molecular structure or small apertures which do not permit the negative pole activating material which precipitates on the negative pole to pass but which permits ions relating to the battery reactions to pass through.
The present invention is characterized in that the foregoing material of the film has been electron donative elements or groups for enabling the ions relating to the battery reactions to be easily conducted in the film.
The electron donative element is exemplified by oxygen atoms, nitrogen atoms, sulfur atoms and transition metal atoms respectively having a paired electron, a non-paired electron or electron d (electrons in the d quantum shell). The electron donative group is exemplified by a ring compound and a compound having a carbon double bond having electron xcfx80 (xcfx80 electron resonance) or an aromatic ring.
The film formed on the surface of the negative pole according to the present invention is characterized in that it cannot be dissolved by the electrolyte.
The inventor of the present invention found that treatment of the surface of the negative pole with a nitrogen compound or a halogen compound, which is active in a gas phase, will prevent the generation of lithium dendrite.
According to the present invention, there is provided a secondary battery having a negative pole activating material composed of lithium, a separator, a positive pole activating material, an electrolyte, a collector and a battery case, wherein at least the surface of the lithium negative pole opposing the positive pole is treated with reactive and gaseous material containing nitrogen or a halogen element.
According to the present invention, there is provided a battery comprising a negative pole, a separator, a positive pole and an electrolyte, wherein one or more layers selected from a group consisting of a conductor layer, a semiconductor layer and an insulating layer are formed between the negative pole and the separator.
If the negative pole activating material is lithium or lithium alloy, the foregoing layer is formed into a micropore structure having small apertures permitting at least lithium ions to pass through. If zinc or zinc alloy is used, the small apertures permit hydride ions to pass through.
The small apertures permitting ions to pass through may be realized by the molecular structure of the material or by a manufacturing method. The small apertures can be easily formed by, for example, injecting an electrolyte into the foregoing layer at the time of forming the layer to manufacture the battery, the electrolyte being eluted to form the micropores. Another method may be employed in which a foaming material is added at the time of forming the foregoing layer and then the micropores are formed by heat treatment or the like.
The structure of the stacked layers may be a single layer or a multi-layer composed of two or more layers or composed of a conductor layer, a semiconductor layer, an insulating layer and a composite layer containing two or more types of elements or compounds.
Further, a fact was found that the separator partially including a film-shaped member (hereinafter sometimes called a xe2x80x9cmetal oxide filmxe2x80x9d) of a metal oxide formed by a mold made of a bimolecular film forming compound is able to prevent short circuits in the battery from occurring between the negative pole and the positive pole even if a dendrite is generated in the negative pole.
According to the present invention, there is provided a secondary battery comprising a negative pole made of a negative pole activating material, a positive pole made of a positive pole activating material and a separator which separating the positive pole activating material and the negative pole activating material from each other, wherein at least a multi-layer metal oxide is present between the positive pole and the negative pole.
Also it was found that an arrangement that surface of the positive pole is covered with a thin film made of an insulating material or a semiconductor which is free from electron conduction and which permits ions relating to battery reactions to pass through will prevent short circuits in the battery between the negative pole and the positive pole even if a dendrite is generated in the negative pole.
According to the present invention, there is provided a secondary battery at least comprising a negative pole, a separator, a positive pole, an electrolyte, a collector and a battery case, wherein at least the surface of the positive pole opposing the negative pole is covered with one or more thin film layers selected from a group consisting of an insulating layer, a semiconductor layer, a layer composed of an insulating material and a semiconductor which permit ions relating to the battery reactions to pass through.
Also it was found that employment of a positive pole activating material 13104 of a lithium secondary battery made of a compound of one or more types of transition metals having a crystal grain size of 500 xc3x85 or less enables high capacity, large energy and long cycle life to be realized. The secondary battery of the foregoing type has a cross sectional shape schematically shown in FIG. 13. The same reference numerals as those shown in FIG. 1 represent the same structures.
According to the present invention, there is provided a lithium secondary battery at least comprising a negative pole activating material, a separator, a positive pole activating material through which ions can be introduced/discharged due to charge/discharge, an electrolyte which is an ion conductor, a collecting electrode and a battery case, wherein the main component of the positive pole activating material 13104 is a compound of one or more type of transition metal and a group VIA element and having a crystal grain size of 500 xc3x85 or less.
The main component material of the positive pole activating material has a structure of an aggregate selected from a group consisting of amorphous, microcrystal, a mixture of amorphous, microcrystal and a mixture of amorphous, microcrystal and multi-crystal.
The arrangement that the positive pole activating material of the lithium secondary battery is made of a compound of the transition metal having a structure of the aggregate selected from a group consisting of amorphous, microcrystal, a mixture of amorphous and a microcrystal and a mixture of an amorphous, a microcrystal and a multi-crystal, and the group VIA element and having a crystal grain size of 500 xc3x85 or less, more preferably 200 xc3x85 enables the following effects to be obtained:
(1) Since the reactive area of the positive pole activating material can be enlarged, the electrochemical reactions at the time of charge and discharge can be made smooth, and therefore the chargeable capacity can be enlarged.
(2) The introduction and the discharge of lithium ions at the time of the charge and the discharge prevent the distortion of the positive pole activating material, causing the cycle life to be lengthened.
It is preferable that the specific area of the positive pole activating material mainly composed of the compound of the transition metal and the group VIA element be 50 m2/g or more in a state before the material is formed into the positive pole, more preferably 100 m2/g or more.
The employment of the compound of the transition metal and the group VIA element containing hydrogen will improve in the charge and discharge cycle characteristics.
By subjecting the positive pole activating material to a lipophilic treatment using an organic metal compound, the solid-liquid reactions between the electrolyte and the positive pole activating material can be made further smooth at the time of charge and the discharge.
The compound of the transition metal and the group VIA element is exemplified by a metal oxide such as a nickel oxide, a cobalt oxide, a titanium oxide, an iron oxide, a vanadium oxide, a manganese oxide, a molybdenum oxide, a chrome oxide or a tungsten oxide, a metal sulfide such as a molybdenum sulfide, an iron sulfide or a titanium sulfide, a hydride such as an oxy iron hydride or their mixtures.
By employment of metal lithium having a film through which lithium ions are able to pass to form the negative pole activating material of the secondary battery, a lithium secondary battery exhibiting a long life and a high energy density can be obtained.
According to the present invention, there is provided a method of manufacturing a positive pole activating material of a lithium secondary battery at least comprising the step of forming a compound of a transition metal and a group VIA element, the raw material of which is one or more types of materials selected from a group consisting of the transition metal, the salt of the transition metal, an organic metal compound of the transition metal, a hydride of the transition metal, transition metal oxide, a carbonyl compound of a transition metal and a transition metal oxide and which has a structure of an aggregate having a crystal grain size of 500 xc3x85 or less, more preferably 200 xc3x85 or less and selected from a group consisting of amorphous, microcrystal, a mixture of amorphous and microcrystal and a mixture of amorphous, microcrystal and multi-crystal.
According to the present invention, there is provided a method of manufacturing a positive pole activating material which is a compound of a transition metal and a group VIA element, the method comprising the steps of:
employing one or more types of reactions selected from a group consisting of a reaction between a salt of the transition metal and alkali, a hydrolysis decomposition reaction of an organic transition metal compound and a reaction between the transition metal and alkali to prepare a hydride of the transition metal;
employing a dehydrating reaction or decomposition of the salt of the transition metal or the organic transition metal compound in a gas phase or a reaction between the salt of the transition metal or the decomposed material of the organic transition metal compound or vapor of the transition metal and the group VIA element or the group VIA compound;
melting one or more types of materials selected from a group consisting of the transition metal and the transition metal compound to be allowed to react with one or more types of materials selected from a group consisting of the group VIA element and the compound of the group VIA element; and
rapidly cooling the materials to form an aggregate having a crystal grain size of 500 xc3x85 or less and formed into a structure selected from a group consisting of amorphous, microcrystal, a mixture of amorphous and microcrystal and a mixture of amorphous, microcrystal and multi-crystal.